Just when you think it's safe to go back in the water

He attacks.

Flash went to the groomer's today and she said he was wonderful and never gives her any trouble...doesn't growl or try to bite. Great news.

Then we came home and Joanne came over to bring me my mail. We were standing in the breezeway and Flash came over. Joanne said something like, "Oh Flash, how pretty you look," putting her hand out to pet him. He gave a low growl and Joanne said something else and then he struck like a snake. Luckily, Joanne was faster. Flash's teeth just grazed her hand.

Then when I grabbed him, he growled at me but didn't try to bite.

Why does he do this?